Field
The present disclosure relates generally to energy storage devices, and more particularly to metal and metal-ion battery technology and the like.
Background
Owing in part to their relatively high energy densities, relatively high specific energy, light weight, and potential for long lifetimes, advanced rechargeable metal batteries and rechargeable metal-ion batteries such as lithium-ion (Li-ion) batteries are desirable for a wide range of consumer electronics, electric vehicle, grid storage and other important applications. Similarly, primary metal and metal-ion batteries, such as primary Li batteries, are desired for a range of applications, where high energy density and/or high specific energy of batteries is needed, even if the batteries may be disposed of after a single use.
However, despite the increasing commercial prevalence of Li-ion batteries and some of the Li primary batteries, further development of these batteries is needed, particularly for potential applications in low- or zero-emission, hybrid-electrical or fully-electrical vehicles, consumer electronics, energy-efficient cargo ships and locomotives, aerospace applications, and power grids.
Conversion-type electrodes, such as metal fluorides, metal chlorides, metal iodides, metal sulfides, sulfur, oxides, metal nitrides, metal phosphides, metal hydrides and others for Li-ion batteries offer high gravimetric and volumetric capacities. In these electrodes, so-called conversion reactions take place when metal ions such as Li are inserted or extracted during battery operation. For example, an iron fluoride (e.g., FeF2) is converted to 2LiF and Fe during an electrochemical reaction of FeF2 with Li ions during Li-ion or Li cell discharge.
Metal fluorides, in particular, offer a combination of relatively high average voltage and high capacities, but suffer from several limitations for various metal-ion (such as Li-ion) battery chemistries. For example, only select metal fluoride particles have been reported to offer some reasonable (although still poor) cycle stability in Li-ion battery cells (specifically AgF2, FeF2, FeF3, CoF2, and NiF2). Many other metal fluorides are generally believed not to be practical for applications in Li-ion batteries due to the irreversible changes that occur in such cathodes during battery operation. For example, during Li-ion insertion into some of the other fluorides (such as CuF2, for example) and the subsequent formation of LiF during the conversion reaction, the original fluoride-forming element (such as Cu in the case of CuF2) produces electrically isolated (Cu) nanoparticles. Being electrically isolated, such nanoparticles cannot electrochemically react with LiF to transform back into CuF2 during subsequent Li extraction, thereby preventing reversibility of the conversion reaction. As a result, after a discharge, the cell cannot be charged back to the initial capacity. In addition to formation of electrically isolated nanoparticles, the irreversible growth of LiF and metal (M) clusters during cycling and the resulting growth of resistance may be yet another serious limitation. This additionally limits the rate performance of such chemistries. Moreover, many attractive (in terms of high theoretical energy density) metal fluorides (such as CuF2) suffer from another degradation mechanism: during Li (or Li-ion) battery operation, the cathode is often exposed to a potential level where a metal (of the corresponding metal fluoride) is oxidized and initially dissolves into the electrolyte, then migrates to the anode and reduces on the anode. This process leads to rapid irreversible capacity losses and cell degradation and may be particularly serious for some of the most otherwise-attractive metal fluoride cathode materials (such as CuF2-based cathodes). Metal chlorides suffer from similar limitations. But in addition, their dissolution during cycling induces formation of Cl-containing ions that corrode cathode current collectors.
Even the cathodes based on those metal fluorides that are believed to be most practical due to their relatively reversible operation and reasonably low cost (such as FeF2, FeF3, CoF2, and NiF2), suffer from multiple limitations including: (i) low electrical conductivity, which limits their utilization and both energy and power characteristics in batteries; (ii) low ionic conductivity, which limits their utilization and both energy and power characteristics in batteries; and (iii) volume expansion during Li-ion insertion, which may cause mechanical and electrical degradation in the electrodes during battery operation.
As a result, despite multiple theoretical advantages of fluoride-based cathodes (and some of the chloride-based cathodes), for example, their practical applications in metal-ion batteries are difficult to achieve. The cells produced with fluoride-based cathodes currently suffer from poor stability, volume changes, slow charging, and high impedance.
Several approaches have been developed to overcome some of the above-described difficulties, but none have been fully successful in overcoming all of them.
For example, decreasing particle size decreases the ion diffusion distance, and offers one approach to addressing the low ionic conductivity limitation. However, nanopowders suffer from high electrical resistance caused by the multiple, highly resistive point contacts formed between the individual particles. In addition, small particle size increases the specific surface area available for undesirable electrochemical (or chemical) side reactions. Furthermore, simply decreasing the particle size does not address and may in some cases exacerbate other limitations of such materials, such as volume changes as well as weakening of the particle-binder interfaces. Moreover, in contrast to using micron-scale particles for cathode formulations, handling nanoparticles and using them to prepare dense electrodes is technologically difficult. Nanoparticles are difficult to disperse uniformly within conductive carbon additives and the binder of the cathode, and an undesirable formation of agglomerates of nanoparticles tends to take place. Formation of such agglomerates reduces the electrode density (thus reducing volume-normalized capacity and energy density of the cells), reduces electrode stability (since the binder and conductive additives do not connect individual particles within such agglomerates) and reduces capacity utilization (since some of the nanoparticles become electrically insulated and thus do not participate in Li-ion storage).
In another approach, select metal fluoride particles which offer some reasonable cycle stability in Li-ion battery cells (specifically FeF2, FeF3, CoF2, and NiF2) may be mechanically mixed with or deposited onto the surface of conductive substrates, such as carbon black, graphite, multi-walled carbon nanotubes, or carbon fibers. In this case, the high electrical conductivity of the carbon enhances electrical conductivity of the electrodes. However, many degradation mechanisms (including those discussed above) are not addressed by this approach. In addition, the phase transformations during battery operation and the volume changes discussed above may induce a separation of the active material from the conductive additives, leading to resistance growth and battery degradation.
In yet another approach, select metal fluoride particles (specifically FeF2 particles) may be coated with a solid multi-walled graphitic carbon shell layer. In this case, the electrical conductivity of a metal fluoride cathode may be improved. However, the above-described volume changes during metal-ion insertion may break the graphitic carbon coating and induce irreversible capacity losses. Similarly, the phase transformation during subsequent charging and discharging cycles may induce a separation of the active material from the graphitic carbon shell, leading to resistance growth and battery degradation. Furthermore, some of the carbon shells are incredibly difficult to deposit on selected metal fluorides (such as copper fluorides) and chlorides due to the simultaneous metal fluoride reduction (for example, reduction of Cu2+ in CuF2 to metallic Cu0)
Accordingly, there remains a need for improved metal and metal-ion batteries, components, and other related materials and manufacturing processes.